Fischer-Tropsch reactor

Shell Gas B.V. has constructed a 1987 mVd (12,500 bbhd) Fischer-Tropsch plant in Malaysia, start-up occurring in 1994. The Shell Middle Distillate Synthesis (SMDS) process, as it is called, uses natural gas as the feedstock to fixed-bed reactors containing cobalt-based cat- yst. The heavy hydrocarbons from the Fischer-Tropsch reactors are converted to distillate fuels by hydrocracking and hydroisomerization. The quality of the products is very high, the diesel fuel having a cetane number in excess of 75. 

The most difficult problem to solve in the design of a Fischer-Tropsch reactor is its very high exothermicity combined with a high sensitivity of product selectivity to temperature. On an industrial scale, multitubular and bubble column reactors have been widely accepted for this highly exothermic reaction.6 In case of a fixed bed reactor, it is desirable that the catalyst particles are in the millimeter size range to avoid excessive pressure drops. During Fischer-Tropsch synthesis the catalyst pores are filled with liquid FT products (mainly waxes) that may result in a fundamental decrease of the reaction rate caused by pore diffusion processes. Post et al. showed that for catalyst particle diameters in excess of only about 1 mm, the catalyst activity is seriously limited by intraparticle diffusion in both iron and cobalt catalysts.1... 

Benham, C. B., Yakobson, D. L., and Bohn, M. S. 2000. Catalyst/wax separation device for slurry Fischer-Tropsch reactor. U.S. Patent 6068760. 

Within each syncrude type some variation is introduced by the operating conditions of Fischer-Tropsch synthesis, such as pressure and H2 CO ratio, as well as by the Fischer-Tropsch reactor type. These variations cannot be ignored, and ultimately they have an impact on the refinery design. During the subsequent discussion it will become apparent that the selection of the Fischer-Tropsch technology influences not only the refinery design, but also the efficiency with which different products can be produced. 

Change occurred in high-temperature Fischer-Tropsch reactor technology. The circulating fluidized bed Sasol Synthol reactors were replaced by fixed fluidized bed Sasol Advanced Synthol (SAS) reactors.44 This did not meaningfully affect the Fe-HTFT syncrude composition, but it reduced the operating cost of HTFT synthesis. 

Gas-to-liquids (GTL) is the chemical conversion of natural gas into petroleum products. Gas-to-liquid plants use Fischer-Tropsch technology, which first converts natural gas into a synthesis gas, which is then fed into the Fischer-Tropsch reactor in the presence of a catalyst, producing a paraffin wax that is hydro-cracked to products (see also Chapter 7). Distillate is the primary product, ranging from 50% to 70% of the total yield. 

Steynberg, A. P., Shingles, T., Dry, M. E., Jager, B., and Yukawa, Y. Sasol commercial scale experience with Synthol [fixed fluid bed] and [circulating fluid bed] Fischer-Tropsch reactors, in Circulating Fluidized Bed Technology HI (P. Basu, M. Horio, and M. Hasatani, eds.), pp. 527-53Z Pergamon, New York, 1991. 

On zeolitic catalysis, secondary reactions have to be anticipated such as isomerization and cracking which can inlluencc the Schulz—Flory distribution. ThLs has been used by the Mobil Oil Corporation to design a dual stage process for conversion of syntltesis gas to a gasoline higl) in octane rating, fn this process, the effluent of a conventional slurry phase Fischer- Tropsch reactor is converted over acidic ZSM-5 zeolite [136). 

Espinoza RE, du Toil E, Santamarfa J, Menendez M, Coronas J, and Irusta S. Use of membranes in Fischer-Tropsch reactors. Stud Surf Sci Catal 2000 130 389-394. 

The reactor operates either at low (200-240°C) or high (300-350°C) temperatures and between 1-4 MPa. The product mix changes from longer to shorter chain length molecules as the temperature increases. The product stream from a Fischer-Tropsch reactor is a mix of many components but by selecting the right operating conditions the mix can be adjusted so that the product stream has mostly diesel fuel properties. 

There are processes in which the total amount of catalyst is entrained by the gas. The reactors then belong to the category of transport reactors. Examples are some of the present Fischer-Tropsch reactors for the production of hydrocarbons from synthesis gas and the modern catalytic cracking units. Fig 10.11 shows the Synthol circulating solids reactor. In the dilute side of the circuit, reactant gases carry suspended catalyst upward, and the fluidized bed and stand-pipe on the other side of the circuit provide the driving force for the smooth circulation of the solid catalyst. For the removal of heat, heat exchangers are positioned in the reactor. 

Fig. 10.11. Synthol circulating fluid bed Fischer-Tropsch reactor. Reprinted from [82] with permission from Elsevier.

The Fischer-Tropsch reactor design challenge can be distilled down to the following critical issues ... 

ISOMERIZATION OF FISCHER-TROPSCH PRODUCT The conditions for operating the tungsten isomerization catalyst are compatible with the composition of the exit stream from a Fischer-Tropsch reactor. The presence of unreacted hydrogen and water vapour together with CO and C02 provides an effective oxygen partial pressure equivalent to that required by the isomerization catalyst. 

P. Schubert, S. LeViness, K. Arcuri, and B. Russell, Historical Development of Cobalt-Slurry Fischer-Tropsch Reactor Systems, available at www.fischer-tropsch.org. 

An analysis of liquid hydrocarbon phase from the Fischer Tropsch reactor and reformer is given in Table XI. As indicated, the Fischer Tropsch product is... 

Table IX. CATALYST SCREENING (Fischer Tropsch Reactor) Ethylene Polymerization...

Figure 5. Temperature vs. yield (Fischer-Tropsch reactor)...

Synthesis gas, commonly known as syngas, is a gas mixture containing CO and H2 and is an important intermediate product for use in Fischer-Tropsch synthesis reactors. The AR generated in z q-zjj space may be useful as a first step to understanding the operating limits of Fischer-Tropsch reactors. 

For all the streams in the PFD the specifications, the composition, the flow rate, and the temperature and pressure, are available. Two options for the streams with CO2 as the major fractions are being evaluated. Option 1 is about the amount of CO2 that can be fed to the Fischer-Tropsch reactor or recycled back to the gas reforming section that produces the syngas. 

Operating Temperature Two modes, low (225-250°C) and high (300-350 °C) temperatures, are used in typical Fischer-Tropsch reactors (Dry 2002). 

Sie ST, Krishna R. (1999) Fundamentals and selection of advanced Fischer-Tropsch reactors. App. Cat. A General, 186 55-70. 

Silverman, R. W., Thompson, A. H., Steynberg, A., Yukawa, Y., and Shingles, T., Development of a Dense Phase Fluidized Bed Fischer-Tropsch Reactor , in Fluidization V (K. Ostergaard and A. Sorenson, eds.). Engineering Foundation, New York, 1986. 

---